The present invention relates to an apparatus for the manufacture of corrugated paperboard and, more particularly, to a heating apparatus for a double facer where a single web is attached to a single face corrugated web.
In a typical prior art double facer, a single web is brought into contact with the glued flute tips of a single face corrugated web and the freshly glued double face web is passed over the surfaces of a number of serially arranged steam chests to cause the starch-based glue to set. Double face web travel over the steam chests is provided by a wide driven holddown belt in direct contact with the upper face of the corrugated web and the top face of the belt held in contact with the traveling web by a series of ballast rollers or the like, all in a well known manner.
Prior art steam chests, one example of which is shown in U.S. Pat. No. 3,175,300, are typically made of heavy cast iron construction in the manner of a pressure vessel in order to contain the high pressure steam which is supplied to the steam chest. For example, the walls of a cast iron steam chest are typically 1" or more thick to safely contain superheated steam supplied, for example, at 350.degree. F. and 160 psi. A steam chest has a flat upper web-supporting surface having a length in a transverse direction sufficient to support the full width of the traveling web and a width in the direction of web movement of typically about 18" to 24". Eighteen steam chests are typically serially arranged in closely spaced relation in a double facer.
The heavy cast iron construction of prior art steam chests results in a number of well known operational problems. The heavy walled construction of these steam chests requires a long time to bring them up to temperature on startup. Eventually, the steam chest may be brought close to the temperature of the steam being supplied to it. However, when operation is commenced and the double face corrugated web is traveling over the upper surfaces of the steam chests, heat is drawn therefrom at a rapid rate and maximum surface temperature may drop to levels as low as 220.degree.-230.degree. F. This lower effective operating temperature may require the use of a substantially larger number of steam chests in a given double facer than would be necessary if more efficient heat transfer were attainable. Another problem directly related to the inefficiency of heat transfer through a heavy iron steam chest casting is the transverse bowing of the upper surface of a conventional steam chest during operation. As indicated, the temperature of the flat upper wall of the steam chest is reduced substantially relative to the bottom wall of the steam chest resulting in a concave bowing of the upper surface lengthwise of the steam chest (transversely across the web traveling thereover). As a result, the holddown belt and transverse ballast rollers pushing the belt downwardly against the upper surface of the web do not impose a uniform load on the web. The result may be uneven curing of the adhesive, zones of poor or no adhesion, and crushing of the lateral edges of the web. Finally, the heavy mass of cast iron steam chests results in high heat retention and slow cool down, often requiring elaborate systems to lift the web or lower the steam chests to avoid excess heating of the web.
U.S. Pat. No. 5,183,525 includes a recognition of certain of the foregoing operational problems in systems utilizing heavy cast iron steam chests. In this patent, the steam chest is replaced by a heavy steel plate through which transverse horizontal bores are drilled and interconnected at their opposite lateral ends to form a serpentine steam passage through the plate. The holes may be drilled in a manner forming a much thinner web of material between the bores and the upper surface of the plate to increase the efficiency of heat transfer. The patent also teaches that the problem of bowing or distortion of the upper contacting face of the plate is minimized. However, the construction of the heating plates in this patent is still quite massive and heavy and, as is well known, the heat transfer efficiency of ferrous metals is relatively poor.
There remains a need, therefore, for a simple, efficient, and low cost hot plate system for a double facer which effectively addresses the problems typical of the prior art.